FIG. 1 presents a simplified block diagram of a conventional digital heterodyne radio receiver 100. The wireless communications signal 90 to be received by such a receiver 100 can be characterized as comprising three (3) components: the desired signal 92, interference 94, and noise 96. The desired signal 92 typically includes information that has been encoded, modulated, and up-converted to a carrier signal in the RF band by a transmitter (not shown).
Conventionally, a carrier signal is a waveform, at a much higher frequency than the baseband-modulated encoded information that is mixed with the modulated encoded information to transmit the information through space as an electromagnetic wave, or to allow several carrier signals at different frequencies to share a common physical transmission medium by frequency division multiplexing. Some types of RF communication, for example, spread spectrum and orthogonal frequency division multiplexing (OFDM), may not use a conventional sinusoidal carrier wave.
The interference 94 is typically from one or more man-made RF sources. Broadly stated, narrowband interference is typically from intended transmissions, such as radio, television, and mobile phone systems; while broadband interference typically is unintentional and emanates from sources such as electrical power transmission lines that are not intended to be transmitters.
Noise 96 refers to all other RF components of energy in the signal 90, of which noise of the receiver itself in the RF range (for example, from a noisy power supply coupled to the RF path) may be a principal component.
In such a receiver 100, the signal 90 can be received by an antenna system 110. Typically, the antenna system 110 is configured to be more responsive to a wide band of frequencies around the carrier frequency, for example, the entire commercial Frequency Modulation (FM) broadcast band comprising many separate FM channels, than to frequencies outside the range of the desired signal 92.
A signal conditioning subsystem 120 typically is used to passively limit noise and interference first by applying an RF Filter 122 to reduce received noise 96 and interference 94 outside the frequency band of the desired signal 92. Then, the filtered signal is amplified, for example, by using a low noise amplifier/variable gain amplifier (LNA/VGA) 124.
After passively reducing noise and interference in the signal conditioning subsystem 120, a conventional heterodyne receiver 100 will remove the carrier, effectively bringing the modulated encoded signal from the RF range, in which it is much easier to transmit the signal over distance, down to baseband. In the case of a simple sinusoidal carrier signal, a mixer 140 mixes the output of the signal conditioning subsystem 120 with a local oscillator (LO) 130 matched to the carrier signal to remove the carrier signal.
In a digital receiver, such as conventional heterodyne receiver 100, the output of the mixer 140 remains an analog signal. Converter 150 is used to convert the analog signal from the mixer 140 to a digital signal. The converted signal can then be processed by digital signal processor 160 to decode the information contained therein.